Coaxial cable switch

ABSTRACT

A high isolation coaxial cable switch is formed by employing a pair of serially connected sealed reed switches with the conducting path between the switches connectable to ground through a third reed switch. The serial switches are make contacts and the ground path switch is a break contact. All switches are operated simultaneously.

Inventors Frank J. Arvay Scotch Plains; Robert A Sutton, Fanwood, both of, NJ. App] No 878,521 Filed Nov. 20, 1969 Patented Aug. 10, 1971 Assignee Bell Telephone Laboratories Incorporated Murray Hill, NJ.

COAXIAL CABLE SWKTCH 5 Claims, 6 Drawing Figs.

[1.8. CI t 335/5, 335/152 lnLCl ..H0lh 51/27 Field ol'Search 335/5,4, 152; ZOO/153.18; 333/7, 97; 307/243 T RT SH! I 51 [5 6] References Cited UNITED STATES PATENTS 3,227 969 1/1966 Bentley it 333/7 3,355.684 11/1967 Church 4 1 .4 335/5 3,422,360 1/1969 Fletcher 307/243 Primary Examiner- Harold Broome Attorneys-11. J. Guenther and Edwin B. Cave All switches are operated simultaneously S2 5H2 T OUT PATENHEB ME I 9 ml SHEET 1 UF 3 |m IS F. J. ARV/1V R A SUTTON /N VE N TORS A TTORNE Y PATENTEU Am; I 0 I97:

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90 Ni 9901 NOIiV'IOSI ovm . Background of the Invention 1. Field of the Invention F This invention relates to high isolation coaxial cable switches and more particularly to those employing sealed reed switches.

2. Description of the Prior Art An important requirement for high frequency communication systems, particularly those employing coaxial cables, is'a switch that provides a high degree of isolation when in the open or unoperative condition. Switches of this general type that employ sealed reed switches are known in the art as shown, for example, by C. B. .Watts, .lr., U.S. Pat. No. 3,194,921 issued July 13, 1965 and by D. S. Church and R. W. Kordos in U'.S. Pat. No. 3,355,684 issued-Nov. 28, 1967. Church et al. for example, show that isolation may be enhanced by connecting three or more'reed switches in series in the transmission path, in contrast to Watts, who discloses the use of a single reed switch witha relatively long cantilever member that operates between two fixed contacts to provide single-pole,double-through operation.

Although it is evident that virtually any desired degree of isolationjmay be achievedin accordance with the teachings of Church et al. by increasing the number of serially arranged reed switches, it is equally clear that'conc'omitant disad vantages arise with such an approach. These disadvantages would include excessive cost, excessive size and reduced reliability. P v e A general object of the inventionis to achieve very high isolation levels in coaxial cable switches'without the disadvantages noted.

Another object is to facilitate low cost assembly of high isolation switches.

Summary of the Invention Thestated objects and additional objects are achieved in accordance with the principles of the invention by a high isolacies, a high degree of isolation is-required in connecting switches.

v BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic circuit diagram of a switch in accordance with the invention;

FIG. 2 is a sketch 'of the circuit component layoutin accordance with the invention; i 7

FIG. 3 is a sketch, partially in cross section of a switch in accordance with the invention;

FIG. 4 is a sketch illustrating the assembly of a switch in accordance with the invention;

FIG. 5 is a sketch of an assembled switch in accordance with the invention; and

FIG. 6 is a plot of isolation loss versus frequency comparing actual and computed characteristics of two different embodition switch that utilizes two reed switches in series relation and a third reed switch connecting ground to a point on the conducting path between the two series switches. All switches are operated simultaneously with the grounding switch being closed whenever the series switches are open. As a result, strayhigh frequency currents that would otherwise be con ducted across by the capacitive coupling afforded by the series switches when open are automatically shunted to ground, and

exceptionally'high level isolation is attained thereby.

In accordance with another feature of the invention, an automatic resistive termination may be provided in the switch for those installations that require it. This feature is achieved by connecting one of the switch terminals to ground by way of the series combination of a fourth reed switch and a resistive element. The fourth switch is made to open simultaneously with the third reed switch that is connected between ground and a point on the conducting path between the two series connected switches.

Another aspect of the invention involves the physical arrangement of the switch that ensures a relatively small, compact and readily assembled structure. All fourswitches are strung together physically in relative series relation although two, as described, are electrically-in shunt with the basic series pair. The switch array is threaded through a nonmagnetic, conductive tube capped at either end by a coaxial plug. The

tube is inserted through a suitable combination of individual ments of the invention.

DETAILED DESCRIPTION an article Recent Developements in Bell System Relays- Particularly Sealed Contact and Miniature Relays," by A. C. Keller, published in the Bell System Technical Journal, Jan. 1964, pages 15-44. Each switch comprises a pair of magnetic reeds, not shown, that are cantilever supported in a hermetically sealed glass envelope. The envelopes of the switches S1, S2, SIM and 8H2 are illustrated in FIGS. 2, 3 and 4. The free ends of the reed overlap, but in the absence of a magnetic field, do not come into contact with one another. In the presence of a magnetic field, however, which may be introduced by direct current applied to the switch coils 31, 32, 33 and '34 by way of a DC control voltage plug 36 as shown in FIG. 3, the free ends of the reeds pull into contact withv one another to form a through circuit. Since the coils 31 through 34 are connected in series, all of the switches are caused to operate simultaneously.

It will be understood that the principles of the invention although illustrated with respect to a coaxial cable are also ap plicable to a noncoaxial transmission line.

From FIG. 1 it is evident that when the switches SI and S2 are open any high frequency leakage current through the inherent capacitance of the switches is shunted to ground by way of the closed switch contact SI-I2. As indicated above, in some arrangements it may be desirable to have a resistive terminal to at least one side of the composite switch, composite as used herein denoting a plurality of contacts. The combination of the switch SH! and resistor R meets this need. When the switches 51 and S2 are closed or conducting, the switch SI-Il is open and the terminating resistance is removed from the circuit.

FIG. 2 indicates, schematically, the physical relation in which the switches are connected in accordance with the invention. As shown, the switches are strung out in serial fashion together'with their electrical interconnections so that the entire assembly of switches 41, as shown in FIG. 4, may readily be threaded through a copper tubing 42. As shown in FIG. 3, the copper tubing is in turn threaded through the switch coils 31 through 34 and through the spacers or sleeves 37, 38 and 39. A circuit component mounting bracket 43 is also slipped on the end of the copper tubing 42 to serve as a mounting for the terminating resistor R, and as a terminal means for the DC control voltage plug 36. At each end the assembly is terminated by one of the coaxial plugs 47 and 48.

The entire assembly is then inserted in a protected sleeve 49 which may be of a phenolic material, for example, and the ends thereof are closed by the cap members 51 and 52. The completed switch in its fully assembled form 50 is shown in FIG. 5.

ln undertaking a theoretical analysis of a switch combination in accordance with the invention, the constants indicated below were employed where the shunt reed switch employed silver tipped paddle contacts or gold tipped paddle contacts, respectively:

SILVER PLA TED PA DDLE TlP Paddle Thickness l l:

Paddle Width l Paddle Length (I;

N,-F, Permeability ml N F, Conductivity 7 Switch Termination tR.)

GOLD PLATED PADDLE TIP 8 x l inches S x 10" inches c 0.8 inches 2.5 X lo" henries/meter l2.56,X l0" henries/meler 2 X It) mhos/meter '7 6 X l0" meters Paddle ThlCkl'W Paddle Width tn y Paddle Length (I) N,-F, Permeability (m) N,-F, Conductivity (T,) Gold Permeability Gold Conductivity (T Plating Thickness (1) Switch Termination (IL) Open contact Switch Capacitance =0.] pf.

A plot of theoretical isolation loss for switch combinations in accordance with the invention as compared to measured values is shown in FIG. 6. It may be noted that at relatively low frequencies the slope of the theoretical curve for the gold reed approaches that of the silver tipped reed (50 db/decade). This relation is caused by the fact that the surface currents on the reed paddles flow into the gold almost undisturbed but are more rapidly attenuated as they penetrate into the N,-F,, material. At higher frequencies, however, the first order skin effect of the gold layer begins to take efiect and combines with the skin effect of the N F to give a 40 db/decade isolation loss slope.

LEQO LOg 9[ As indicated in FIG. 6 the gold plated reeds provide a 16 db. improvement in isolation loss at 10 MHz. In one specific embodiment of the invention employing silver tipped reeds as the series switching elements and gold plated reeds for the shunt elements, the measured isolation loss at 10 MHz was l62 db.

It is to be understood that the embodiment described herein is merely illustrative ofthe principles of the invention. Various modifications thereto may be effected by persons skilled in the art without departing from the spirit and scope of the invention What we claim is: i

1 High isolation switching apparatus comprising, in combination. an input and an output point, means including first and second reed switches in series relation connecting said points, means including a third reed switch connecting a point on the conducting path between said first and second switches to a point of reference potential, operatingmeans simultaneously opening said first and second switchesand closing said third switch. and for simultaneously closing said first and second switches and opening said third switch. h

2. Apparatus in accordance with claim} including a switch termination means which comprises a fourth reed switch and a resistive circuit element connected in series relation between said in ut point and said point of referencepotential. A

3. pparatus in accordance with claim 2 wherein sa d fourth switch is connected for synchronous operation by said operating means with said third switch.

4. High isolation switching apparatus for effecting a switchable connection between coaxial cables comprisin'g'fin combination, an input point and an output point, means including a center conductor and a pair of reed switches connected therein in serial relation, a third reed switch connectable between a point on said center conductor located between said pair of switches and a point of reference potential, a nonmagnetic outer conductor surrounding said center conductor and said switches, a plurality of coils connected in series relation, each surrounding said outer conductor at a point corresponding to a respective one of said switches, means applying a direct current to said coils thereby to operate said switches simultaneously, said pair of switches operating in a closure mode opposite to said third switch, a protective sleeve member surrounding said outer conductor, toroidal end closure members fitted over the ends of said sleeve member, and an input and an output coaxial plug each connected to Said input and output points, respectively.

5. Apparatus in accordance with claim 3 including a switch termination means which comprises a fourth reed switch and a resistive circuit element connected in series relation between said input point and said point of reference potential, and a fourth coil connected in series relation with said plurality of coils for operating said fourth switch in mode and time synchronism with said third switch. 

1. High isolation switching apparatus comprising, in combination, an input and an output point, means including first and second reed switches in series relation connecting said points, means including a third reed switch connecting a point on the conducting path between said first and second switches to a point of reference potential, operating means simultaneously opening said first and second switches and closing said third switch, and for simultaneously closing said first and second switches and opening said third switch.
 2. Apparatus in accordance with claim 1 including a switch termination means which comprises a fourth reed switch and a resistive circuit element connected in series relation between said input point and said point of reference potential.
 3. Apparatus in accordance with claim 2 wherein said fourth switch is connected for synchronous operation by said operating means with said third switch.
 4. High isolation switching apparatus for effecting a switchable connection between coaxial cables comprising, in combination, an input point and an output point, means including a center conductor and a pair of reed switches connected therein in serial relation, a third reed switch connectable between a point on said center conductor located between said pair of switches and a point of reference potential, a nonmagnetic outer conductor surrounding said center conductor and said switches, a plurality of coils connected in series relation, each surrounding said outer conductor at a point corresponding to a respective one of said switches, means applying a direct current to said coils thereby to operate said switches simultaneously, said pair of switches operating in a closure mode opposite to said third switch, a protective sleeve member surrounding said outer conductor, toroidal end closure members fitted over the ends of said sleeve member, and an input and an output coaxial plug each connected to said input and output points, respectively.
 5. Apparatus in accordance with claim 3 including a switch termination means which comprises a fourth reed switch and a resistive circuit element connected in series relation between said input point and said point of reference potential, and a fourth coil connected in series relation with said plurality of coils for operating said fourth switch in mode and time synchronism with said third switch. 